The present invention relates to energy generation systems and, more particularly, to energy generation systems which rely on a controlled particle flux to sustain the generation process. A major objective of the present invention is to improve the stability and effectiveness of neutron absorption in control rods used to regulate energy generation in a nuclear reactor.
Nuclear reactors rely on a controlled, self-perpetuating neutron flux to sustain fission. In a nuclear reactor, neutrons scatter in the uranium-or-plutonium-based fuel rods causing fission. This process generates energy in the form of heat, as well as the additional neutrons needed to keep the reaction self-sustaining. However, since more than one neutron is generated when a uranium or plutonium fission occurs, control rods are used to absorb excess neutrons to keep the reaction in a steady state. Control rods inserted more deeply into the reactor core can absorb enough neutrons to turn the reactor off.
Control rod movement is used to control the reactivity ramp rate. There is a control rod position which maintains the power level constant. By controlled withdrawal of the control rod from this position, the power can be ramped up. When the desired power level is reached, the control rod is inserted back into the constant power position. Alternatively, the control rod can be inserted beyond the constant power level position to ramp down the power. The distance of the control rod from the constant power position determines the reactivity ramp rate.
Conventionally, control rods include a material that has a high absorption cross-section to the neutrons in the reactor core. In particular, a material with a high cross-section to neutrons having the specific momentum that enables them to cause fission in the fuel rods is employed. To be effective, it must absorb the neutron flux of "fission causing" neutrons. Typical control rods comprise a number of parallel hollow tubes filled with a neutron-absorbing material such as boron carbide, hafnium, cadmium, gadolinium, europium, erbium, samarium, dysprosium, silver and/or indium.
Nuclear reactors can be classified according to the method used to transfer fission-generated heat from the reactor core. In boiling-water reactors (BWRs), water is converted to steam as it flows through the core. The steam can be conveyed from the reactor vessel enclosing the core to a turbine. The steam drives the turbine which, in turn, drives a generator to produce electricity.
In addition to serving as the source of steam used to drive the turbine, the water serves as a neutron moderator. High-energy, or "fast" neutrons released during a fission reaction are moderated, i.e., slowed, as they scatter off the hydrogen atoms in the water. Neutron moderation is used to facilitate a chain reaction by slowing neutrons to a momentum at which they can be more easily absorbed by fissionable materials in the fuel elements. Likewise, neutron moderation facilitates control of fission since the slowed neutrons are more readily utilized by the Absorber material in the control rods.
Some reactors have employed control rods utilizing a "flux-trap" design that takes greater advantage of the neutron moderating effect of water. Flux-trap control rods use hollow rather than solid absorber tubes of neutron-absorbing material. The neutron absorption is most effective at water-absorber boundaries. From the perspective of an individual neutron, the more water-absorber interfaces it encounters, the more likely it will be absorbed. A neutron passing through a solid absorber rod passes from water to absorber at most once. A neutron passing through a hollow tube of absorber material can pass through two water-to-absorber boundaries, and thus has a greater chance of being moderated and consequently absorbed. The additional neutrons absorbed at the interface in the internal part of the tube lead to the term "flux-trap".
Typically, the absorption efficiency of a hollow absorber tube is ten to twenty percent higher than it is for a solid absorber rod having the same outer diameter. Since a hollow tube includes less material than a solid rod of the same outer diameter, the flux-trap design provides tubes which require less absorber material. Since absorber materials, notably hafnium, tend to be heavy and expensive, the flux-trap design provides for control rods which are less expensive and more efficient.
One problem encountered when employing flux-trap absorber tubes is that their effectiveness decreases with increasing volume of steam in their interiors. The energy the neutrons impart to the absorber tubes is mostly turned into heat, raising their temperature to around 650-700 degrees Fahrenheit. Since the pressure inside the core is close to 1000 pounds per square inch the reactor water will not boil until its temperature is about 550 degrees Fahrenheit. Water flow inside the core is such that the water comes in from the bottom, and flows upwards in a direction parallel to the absorber tubes. The water flowing external to the absorber tubes travels fast enough past the tubes so it does not boil except for an acceptable amount of surface nucleate boiling. However, the water traveling up the inside of the tubes goes slower than the water external to the tubes, and is more susceptible to boiling from the heat generated in the hafnium.
If the water boils, the volume of water is displaced by the generated steam. Steam is a less effective moderator than water because it provides fewer scattering targets per unit volume. Neutron moderation and, thus, absorption efficiency decreases with increased steam. While this loss of efficiency is undesirable, of even greater concern are the fluctuations in moderation that can occur, as the amount of steam in the tubes can vary rapidly near the water's boiling point.
To alleviate this problem, holes are made in the absorber tube to allow an exchange of water between the inside and outside of the tube. Absorber tubes of this type are described in U.S. Pat. No. 4,882,123. The holes permit relatively cool water from the exterior of the tube to transfer to the interior of the tube so that the tendency to boil is reduced. However, at higher power levels, an undesirable level of interior boiling still occurs. Accordingly, to enhance power control at higher reactor power output levels, flux-trap absorber tubes are desired which more effectively minimize interior boiling.